A telephone is generally connected, by means of a telephone line, to an exchange which supplies the energy necessary to operate the telephone. In particular, the exchange constantly transmits a direct-current supply voltage on the telephone line. During a conversation, the direct-current voltage operates the various conversation members of the telephone such as, for example, a microphone and a loudspeaker, which are housed in a handset. On the other hand, when the telephone receives an incoming call, the exchange also transmits a corresponding sinusoidal signal on the telephone line; this call signal operates a buzzer which informs a user of the incoming call.
In a known passive telephone, there is a double switch operated by a fork-shaped hook for the handset. When the handset is resting on the fork-shaped hook, the switch connects the buzzer to the telephone line (in series with a decoupling capacitor); when the handset is lifted, the switch connects the conversation members to the telephone line. The call signal transmitted by the exchange on the telephone line thus rings the buzzer directly until the user lifts the handset in order to answer the call.
In a known electronic telephone, on the other hand, the buzzer and the conversation members are controlled by respective control units. When the telephone receives an incoming call, the signal transmitted by the exchange on the telephone line is used by an alternating-current/direct-current (AC/DC) converter to supply the control unit of the buzzer. When the user lifts the handset in order to answer the call, a switch operated by the fork-shaped hook connects a direct-current/direct-current (DC/DC) converter to the telephone line; this DC/DC converter supplies the control unit of the conversation members (whilst the AC/DC converter is automatically deactivated).
Each time the user lifts the handset in order to answer a call and thus activates the DC/DC converter, the control unit of the conversation members is subject to an abrupt transient phenomenon before reaching a steady operating state. During the transient phenomenon, electrical interference with components having frequencies within the acoustic band is produced and turns into annoying noises in the loudspeaker of the telephone.
In order to solve this problem, it has been proposed to use filters which reduce the effect of the electrical interference. These filters require the use of fairly bulky, discrete components which have to be disposed outside an integrated circuit in which the control unit of the conversation members is typically formed.
A further problem is that it is necessary to keep the duration of the transient phenomenon within a narrow limit to prevent annoying waiting times for the user. A known solution is to provide speed-up circuits for activating the DC/DC converter, which reduce the duration of the transient phenomenon. However, these speed-up circuits are quite complex and rather expensive.
With reference in particular to FIG. 1, this shows a fixed telephone 100 commonly used in private dwellings and in offices. The telephone 100 has a main body 105 made, for example, of plastics material. A handset 110 which houses a microphone and a loudspeaker is connected to the main body 105 by means of a flexible cable wound in a spiral. The handset 110 normally rests on a fork-shaped hook 115 which projects from a seat formed in the main body 105. The telephone 100 also has a keypad 120 which is used for dialing a telephone number to be called. Within the main body 105 there is a buzzer 125 which communicates with the exterior through a perforated portion. A connector 130 (connected to the main body by means of a flexible cable) is used to connect the telephone 100 to a telephone socket (not shown in the drawing).
In a known telephone, as shown in FIG. 2, the connector has two terminals 130a and 130b which are connected to a telephone line 205. The terminals 130a, 130b are connected to respective input terminals of a polarity conditioning circuit 210 typically formed by a bridge composed of diodes with low conduction thresholds or a bridge composed of MOS transistors; the polarity conditioning circuit 210 has a reference output terminal 215g and a supply output terminal 215v. 
The telephone includes a supply constituted basically by an alternating-current/direct-current (AC/DC) converter which absorbs energy from the telephone line 205. The supply comprises an input section (AC/DC) 220 having a reference terminal connected to the terminal 215g and an input terminal connected to the terminal 215v. The AC/DC input section 220 has an output terminal connected to an anode terminal of a diode D1. A cathode terminal of the diode D1 is connected to a terminal of a capacitor C1 the other terminal of which is connected to the reference terminal 215g. A comparator block 225 receives as an input a signal which is present at the supply terminal 215v; the comparator block 225 outputs a call presence signal R_ON which is supplied to an enabling terminal of the AC/DC input section 220.
The supply 220, D1, C1 described above supplies a control unit (CTRL) 230 of the buzzer 125. The control unit 230 has a reference terminal and a supply terminal which are connected, respectively, to the terminal 215g and to the cathode terminal of the diode D1. The control unit 230 also receives, at its own enabling terminal, the call presence signal R_ON supplied by the comparator block 225. The control unit 230 has two output terminals which are connected to respective terminals of the buzzer 125.
The telephone has a further supply (which absorbs energy from the telephone line 205), constituted substantially by a direct-current/direct-current (DC/DC) converter. This supply includes an input section (DC/DC) 240 having a reference terminal connected to the terminal 215g and an input terminal connected to the terminal 215v by means of a switch 245 operated by the fork-shaped hook 115 on which the handset 110 rests. The DC/DC input section 240 has an output terminal connected to an anode terminal of a diode D2. A cathode terminal of the diode D2 is connected to a terminal of a capacitor C2 the other terminal of which is connected to the reference terminal 215g.
The further supply 240, D2, C2 described above supplies a control unit (CTRL) 250 for the microphone and the loudspeaker which are housed in the handset 110. The control unit 250 has a reference terminal and a supply terminal which are connected, respectively, to the terminal 215g and to the cathode terminal of the diode D2. The control unit 250 has two output terminals which are connected to respective terminals of the handset 110.
The telephone constantly receives a direct-current supply voltage from the telephone line 205. For example, a battery disposed in a telephone exchange supplies to the telephone line 205 a supply voltage equal to 48V; because of the voltage drop in the telephone line 205 (which has a length of up to a few km), a supply voltage of the order of 4-10V (of positive or negative value according to the way in which the terminals 130a, 13Gb are connected to the telephone line 205) is available at the terminals 130a, 130b. The polarity conditioning circuit 210 ensures that a voltage of positive value relative to the reference terminal 215g (substantially equal to the absolute value of the voltage at the terminals 130a, 130b minus the voltage drops in the diodes of the bridge) is always available at the supply terminal 215v. 
In a rest condition, the call presence signal R_ON is deasserted so that the AC/DC input section 220 is deactivated. At the same time, the handset 110 is resting on the fork-shaped hook 115 and the switch 245 is therefore open.
When the telephone receives an incoming call, the exchange transmits on the telephone line 205 a sinusoidal call signal having, for example, a peak to peak amplitude of 250 V and a frequency of 100 Hz. The comparator block 225 detects the presence of the call signal and asserts the call presence signal R_ON. The AC/DC input section 220 is consequently activated and charges the capacitor C1 to an internal supply voltage, for example, equal to 2.5-3V (the diode D1 ensures that the capacitor C1 cannot be discharged to the AC/DC input section 220). The voltage at the terminals of the capacitor C1 supplies the control unit 230 (providing a relatively high power). The control unit 230 is activated by the call presence signal R_ON and therefore causes the buzzer 125 to ring.
As soon as a user lifts the handset 110 in order to answer the call, the fork-shaped hook 115 causes the switch 245 to close. The exchange notices a change in current absorption on the telephone line 205 by the telephone and therefore interrupts the call signal and establishes a connection with a calling telephone. The call presence signal R_ON is consequently deasserted so that the AC/DC input section 220 is deactivated. The DC/DC input section 240 charges the capacitor C2 to the internal supply voltage (the diode D2 ensures that the capacitor C2 cannot be discharged to the DC/DC input section 240). This voltage at the terminals of the capacitor C2 supplies the control unit 250 (supplying a fairly low power). This enables a conversation to be activated by the transmission, on the telephone line 205, of a signal representative of voice data and having a limited peak to peak amplitude (for example of a few V) and a frequency within the acoustic band (200-3,400 Hz).
In some known telephones, the control unit 230 and the buzzer 125 are also used during conversation, for example, with a hands-free function. In this case, the DC/DC input section 240 also supplies the control unit 230 by means of a further diode and a further capacitor (not shown in the drawing).
Each time the DC/DC input section 240 is activated, the charging time of the capacitor C2 gives rise to a transient phenomenon in the supply voltage of the control unit 250 with consequent electrical interference with components having frequencies within the acoustic band. To prevent this electrical interference from being translated into annoying noises in the loudspeaker housed in the handset 110 and to reduce the duration of the transient phenomenon, the telephone is provided, respectively, with filters and with speed-up circuits for charging the capacitor C2 (not shown in the drawing).